1. Industrial Field of Application
The present invention relates to a polymer dispersed liquid crystal electro-optical device comprising a liquid crystal/resin composite composed of a high molecular resin having dispersed therein a liquid crystal material. More particularly, it relates to a liquid crystal electro-optical device having a high scattering efficiency.
2. Prior Art
Conventional liquid crystal electro-optical devices include the well known and practically used devices operating in a TN (twisted nematic) or an STN (super twisted nematic) mode. These liquid crystal electro-optical devices are based on nematic liquid crystal and the like. Furthermore, devices using ferroelectric liquid crystals have recently come to our knowledge. Those known liquid crystal electro-optical devices basically comprise a first and a second substrate each having established thereon an electrode and a lead, and a liquid crystal composition being incorporated therebetween. By taking this assembly, the state of the liquid crystal molecules can be varied by applying an electric field to the liquid crystal composition, because the liquid crystal material itself has an anisotropy in dielectric constant, or, in the case of a ferroelectric liquid crystal, it exhibits spontaneous polarization. The electro-optical effect which results therefrom is made use of in the aforementioned liquid crystal electro-optical devices.
In a liquid crystal electro-optical device operating in a TN or STN mode, an alignment treatment, i.e., rubbing, is applied to align the liquid crystal molecules along the rubbing direction at each of the planes in contact with the two substrates by which the liquid crystal layer is sandwiched. Rubbing is applied to the upper and the lower planes as such that their directions may be displaced by 90° or by an angle between 200° and 290° from each other. Accordingly, the intermediate liquid crystal molecules in the liquid crystal layer, i.e., those between the upper and the lower molecules positioned at an angle of from 90° to 290° adjacent to the substrates arrange themselves into a spiral to achieve a configuration of lowest energy. In the case of an STN type liquid crystal device, a chiral substance is optionally added to the liquid crystal material if necessary.
The aforementioned devices, however, require polarizer sheets to be incorporated. Moreover, the liquid crystal molecules need to be regularly arranged in the liquid crystal electro-optical device to achieve a predetermined alignment. The alignment treatment as referred herein comprises rubbing an alignment film (ordinarily an organic film) with a cotton or a velvet cloth along one direction. If not for this treatment, the liquid crystal molecules are unable to attain a predetermined alignment, and hence, no electro-optical effect can be expected therefrom. Accordingly, conventional liquid crystal electro-optical devices above unexceptionably comprise a pair of substrates which make a container to hold therein a liquid crystal material. Then, the optical effect which results from the oriented liquid crystal having charged into the container can be utilized.
There is also known another type of liquid crystal, a polymer dispersed liquid crystal (referred to sometimes hereinafter as PDLC), which can be used without incorporating any polarizer sheets and applying an alignment treatment and the like. In FIG. 7 is shown schematically a PDLC. A PDLC electro-optical device comprises a solid polymer 4 having dispersed therein a granular or sponge-like liquid crystal material 3 between a pair of light-transmitting substrates 1 to give a light-control layer. A liquid crystal device of this type can be fabricated by dispersing microcapsules of a liquid crystal material in a polymer, and then forming a thin film thereof on a substrate or a film. The liquid crystal material can be encapsulated using, for example, gum arabic, poly(vinyl alcohol), and gelatin.
In the case of liquid crystal molecules being encapsulated in poly(vinyl alcohol), for example, if they show a positive dielectric anisotropy in the thin film, an electric field may be applied in such a manner that their major axes may be arranged in parallel with the electric field. Accordingly, a transparent state can be realized if the refractive index of the encapsulated liquid crystal is equal to that of the polymer. When no electric field is applied, the liquid crystal microcapsules take a random orientation and the incident light is scattered because the refractive index of the liquid crystal greatly differs from that of the polymer. Thus, an opaque or a milky white state is realized. In FIG. 8 is shown the change of transmittance in relation with the applied voltage in the liquid crystal electro-optical device above. The transmittance changes with increasing and decreasing voltage as indicated with arrows in the figure. If the liquid crystal microcapsules have a negative dielectric anisotropy and if the average refractive index of the liquid crystal is equal to that of poly(vinyl alcohol), a transparent state can be realized by applying no electric field.
The term “average refractive index” as referred herein is defined as follows. When no electric field is applied to a liquid crystal material on a non-treated substrate, the refractive indices thereof are found to be distributed as shown in FIG. 10. In the figure, no and ne represent the refractive index for an ordinary light and an extraordinary light, respectively. The “average refractive index” is then defined as the index nave at the maximum distribution intensity in the curve as shown in FIG. 10.
In the presence of an electric field, on the other hand, a milky white or an opaque state results, because the liquid crystal molecules are arranged as such that the major axes thereof make a right angle with respect to the direction of the electric field to thereby develop a difference in refractive index. A similar result is obtained if the liquid crystal molecules themselves exhibit spontaneous polarization along a direction vertical to the major axes of the liquid crystal molecules. In such a case, the transmittance changes with increasing or decreasing voltage as shown in FIG. 9. In this manner, a PDLC electro-optical device provides various types of information by making the best of the difference between the transparent and the opaque state.
Polymer dispersed liquid crystals include not only those of the encapsulated type, but also those comprising liquid crystal materials being dispersed in an epoxy resin, or those utilizing phase separation between a liquid crystal and a resin which results by irradiating a light for curing a photocurable resin being mixed with a liquid crystal, or those obtained by impregnating a three-dimension polymer network with a liquid crystal. All those enumerated above are referred to as “polymer dispersed liquid crystals” in the present invention.
Because the electro-optical devices using PDLCs are free of polarizer sheets, they yield a far higher light transmittance as compared with any of the conventional electro-optical devices operating on TN mode, STN mode, etc. More specifically, because the light transmittance per polarizer sheet is as low as about 50%, the light transmittance of an active matrix display using a combination of polarizer sheets as a result falls to a mere 1%. In an STN type device, the transmittance results as low as 20%. Accordingly, an additional backlighting is requisite to compensate for the optical loss to lighten those dark displays. In the case of a PDLC electro-optical device, by contrast, 50% or more of light is transmitted. This is clearly an advantage of a device using no polarizer sheets.
Because a PDLC takes two states, i.e., a transparent state and an opaque state, and transmits more light when used in a liquid crystal electro-optical device, the R & D efforts are more paid for developing devices of a light transmitting type. More specifically, particular notice is taken to a light-transmitting liquid crystal electro-optical device of a projection type.
A projection type liquid crystal electro-optical device comprises placing the liquid crystal electro-optical device panel on a light path of a light beam being generated from a light source, and then projecting the light against a flat panel through a slit being provided at a predetermined angle. If the liquid crystal molecules in the liquid crystal panel have a positive dielectric anisotropy, they take a random orientation to realize an opaque (milky white) state in the low electric field region; i.e., at any voltage below a threshold voltage at which the liquid crystal molecules do not respond to the applied voltage. The light incident to a panel at such a state is scattered to widen the light path. The light having scattered then proceeds to the slit, but most of them are cut off to yield a dark state on the flat panel.
On the other hand, when the liquid crystal molecules respond to the applied electric field and when they are thereby arranged in parallel with the direction of the electric field, a light incident thereto passes straight forward to yield a bright state at a high contrast on the flat panel. When the liquid crystal molecules have a negative dielectric anisotropy, or when they have spontaneous polarization along a direction vertical to the major axes of the molecules, and if the average refractive index of the liquid crystal molecules coincide with that of the polymer resin matrix, the liquid crystal electro-optical device panel turns transparent when no electric field is applied; it reversely turns opaque to yield a dark state by scattering the light when an electric field is applied.
As described in the foregoing, the switching of states of a PDLC occurs, in principle, by the scattering of light. That is, in passing through the light control layer comprising the resin and the liquid crystal droplets which differ from each other in terms of refractive index, the light incident on the transparent substrate side greatly changes its course each time it comes to the boundary between the resin and the liquid crystal. Accordingly, the incident light reaches the substrate on the other side in a completely scattered state. To increase the scattering efficiency of the light control layer, it is preferred that the liquid crystal droplets are more frequently brought into contact with the resin along the thickness direction of the light control layer. The more the boundary between a resin and a liquid crystal droplet is provided for a light, the more scattered the light becomes. Accordingly, the scattering efficiency can be increased by providing a thicker control layer. However, a thicker control layer adversely increases the spacing between the substrates, that is, the distance between the electrodes. A longer distance between the electrodes require a larger driving voltage for switching the light control layer. This makes it impossible to drive the liquid crystal cell with an ordinary IC (integrated circuit), particularly with a TFT (thin film transistor).
A practically feasible liquid crystal electro-optical device should, in general,    1) be driven at a low voltage;    2) have rapid response; and    3) be driven at a speed of 0.1 msec (100 μsec) or less even in a cell having a thickness in the range of from 2.5 to 10 μm.
Most of the conventional PDLC electro-optical devices are based on a nematic liquid crystal material, and are yet to satisfy the required quick response. No liquid crystal electro-optical device which satisfy all of the requirements enumerated above and still capable of providing a rapid optical response to dynamic displays without using any polarizer sheets is proposed to present. However, a PDLC electro-optical device using a ferroelectric liquid crystal material is known as a device which satisfy a part of the requirements above. This type of liquid crystal electro-optical device, however, because of the ferroelectric nature of the liquid crystal, exhibits a piezoelectric effect while it is driven. More specifically, the liquid crystal being incorporated between the electrodes undergoes shrinking by the electric field being applied for driving the liquid crystal, and such a change in volume initiates vibration of the substrates as a source of noise. Furthermore, such a vibration of the substrates my cause damage to the liquid crystal electro-optical device due to the peeling off which occurs on the pair of substrates which are adhered to make a cell.